1. Field of the Invention
This invention relates to a proppant, to a method for producing the proppant, and to a method for propping a subterranean well with the proppant. Various particulate materials, including sand, glass spheres, bauxite and various substrates coated with a fusible thermo-setting resin have been suggested for use as proppants. Such use involves, for example, injecting a fluid into a well to form a fracture in a subterranean formation which the well penetrates and injecting a proppant suspended in a carrier fluid into the formation. The proppant, when so injected, is deposited in the fracture to "prop" the adjacent walls apart so that the fracture is not closed by the forces which are extant in the subterranean formation.
2. Description of the Prior Art
In general, siliceous sand, because it is widely available, inexpensive, and comparatively effective, is an ideal proppant, at least for use in comparatively shallow formations. It has been found, however, that sand is not a desirable proppant when used in formations sufficiently deep that the fracture is subjected to closure stresses exceeding about 5,000 p.s.i.. At greater stresses, it is believed, the sand particles are disintegrated, forming fines which then plug the formation, reducing its permeability. This is discussed in U.S. Pat. No. 3,929,191, granted Dec. 20, 1975, to Graham, et al.
Sintered bauxite has also been suggested as a proppant, and is preferable to siliceous sand because of its ability to withstand higher stresses without disintegration. However, sintered bauxite is less desirable than siliceous sand as a proppant because it is substantially more expensive and is less generally available. The use of sintered bauxite as a proppant is disclosed in U.S. Pat. No. 4,068,718, granted Jan. 17, 1978, to Cook, Jr., et al.
The previously identified Graham, et al, patent also discloses the use, as a proppant, of sand coated with "a powdered phenol formaldehyde resin sold by Union Carbide Corporation under the tradename BRPE 4035 one-step, thermosettable phenolic resin". When sand coated with the "powdered phenol formaldehyde resin" is placed in a subterranean formation, the Graham, et al. patent indicates, the heat of the formation causes the resin coating to melt and, ultimately, to cure; while the coating is melted, coalescence occurs between the coatings on adjacent particles so that, after cure of the resin coating, many sand particles are bonded together thereby. The Graham, et al. patent also indicates that various substrates, particularly glass beads, coated with thermosettable resins, are also suitable proppants, but that substrates, specifically sand, coated with the BRPE 4035 material and "pre-cured" are actually less advantageous than the uncoated substrates.
As has been indicated above, the function of a proppant is to "prop" the walls adjacent a fracture in a subterranean formation apart so that the fracture is not closed by the forces which are extant in the formation. It is advantageous for the walls adjacent the fracture to be "propped" apart so that the formation can be worked, usually to remove oil or natural gas. It will be appreciated that the coalescence which occurs after the Graham, et al. coated particles are placed, as described above, reduces the porosity within the fracture, correspondingly interfering with the ultimate working of the well. It has also been found that before sand cures to a thermoset condition, sand and other particulate materials present in the subterranean formation can contaminate the proppant by adhering to the sand coated with the resin, still further reducing porosity and often constituting the weakest links in the proppant "chain", collapsing and breaking into smaller particles under pressures which the proppant, if not contaminated, could withstand without deteriment. It has been found that this latter phenomenon causes fractures propped with particulate matter coated with a fusible, synthetic resin to become plugged much sooner than would be predicted on the basis of permeability measurements made at various applied stresses in a comparatively clean environment. It has also been found that the BRPE 4035 material is a blend of 1/3 part of a resole and 2/3 part of a novolac, and not the simple "one step, thermosettable phenolic resin" it is characterized by said Graham, et al. patent as being.